The eukaryotic cytochromes P450 are membrane-associated enzymes that catalyze a wide range of monooxygenase reactions. Human cytochromes P450 are responsible for many synthetic and degradative oxidation reactions with substrates that include fatty acids, prostaglandins, exotins and steroid hormones. While the crystal structures of bacterial cytochromes P450 have been determined, there is little sequence identity between the prokaryotic and eukaryotic forms, and the former are not membrane enzymes. We have crystallized a form of mammalian cytochrome P450 that diffracts X-rays to 3.0 E resolution.